Sputter coating is a process where atoms of a solid target are ejected by the bombardment of energetic ions onto the target. The collection of these sputtered atoms on a nearby object, called a substrate, coats the substrate with the target material. The source of the bombarding ions is commonly a gas discharge, where collisions between electrons and neutral gas atoms results in the generation of electron and gas ion pairs, the ions having a positive charge. A negatively charged electrode (cathode) placed in the gas discharge attracts the positive ions causing the ion bombardment responsible for sputtering.
The target is consumed by the sputtering process and requires periodic replacement. A cathode assembly supports the target, provides water cooling, sets up a magnetic field in the region of the gas discharge and shields non-target portions of the cathode from unwanted ion bombardment. Most sputtering systems operate with the target at a negative potential, with a grounded metal chamber acting as an anode. The gas discharge is usually made from argon gas at pressures in the range of 1 to 20 millitorr. (Atmospheric pressure is 760 Torr).
Argon is the gas of choice because of its chemical inertness, relatively large atomic mass, and relatively low cost. Electrical gas discharges can be achieved with any gas, but if a chemically reactive gas is chosen, it will react with atoms sputtered from the target to yield a coating which is the reaction product of the two constituents. When this is intentionally done, the process is termed reactive sputtering. An example of reactive sputtering is the sputtering of a titanium target in a nitrogen-argon gas mixture to yield a coating of titanium nitride. Residual atmospheric gas contaminants present in the gas discharge will also react with the coating material resulting in its contamination. Since this is to be avoided, many sputtering systems are evacuated in the region of the discharge to pressure levels of 1.times.10.sup.-7 Torr or less prior to introduction of the ion providing gas.
Another method of minimizing coating contamination is achieved by increasing the coating rate. This method is effective because at a given constant residual gas pressure, the degree of coating purity is directly proportional to coating rate. Thus, a doubling of the coating rate has the same effect as halving the residual gas pressure.
Many sputter coating applications require that the temperature of the substrate be regulated to achieve optimum coating quality. The substrate support is accordingly provided with a heating and/or cooling means. Similarly, coating quality can also be improved if the substrate is subjected to a moderate negative bias with regard to the gas discharge. This method, termed bias sputtering, causes positive argon ion bombardment of the coating during its growth, which bombardment can have beneficial effects on coating porosity, stress and conformality.
The sputtering process may be used as a material removal or surface cleaning method. For this use the discharge chamber is not equipped with a sputtering cathode. Instead, the object to be cleaned or etched becomes part of the primary system cathode. The ensuing ion bombardment of the substrate removes surface contaminants and may also remove some of the bulk atoms of the object. This process is termed sputter-etching and is frequently used in a sputtering system as a preparatory step prior to the deposition of target material onto a substrate.
A measure of transport efficiency for a sputter coating target is the fraction of the material sputtered off the target which reaches the substrate that is being coated. Thus, 50% efficiency represents the condition where half the material which sputters off the target due to ion bombardment coats the substrate. The remaining sputtered material usually coats other parts of the sputtering process chamber. There are many reasons why it is very desirable to increase transport efficiency.
Firstly, many sputter coating materials are costly and since the unused sputtered material cannot effectively be recovered, this represents a significant additional coating cost.
Secondly, the unused sputtered material usually coats other sputter chamber components where, after a time, it causes problems such as particulate generation or electrical shorting of insulators. Consequently, the coating process must be interrupted for purposes of removing this extraneous material. Such maintenance typically requires that many hours elapse before coating can resume and therefore also represents a significant operating cost.
Thirdly, typical target volumes, being limited by cathode design constraints, require that the sputter coating process be interrupted for purposes of spent target replacement. Here again, the coating process is disrupted.
Fourthly, the deposition rates achievable with sputtering processes are inherently low, and are frequently limited by the size and cost of available power supplies, or limited by heat dissipation factors inherent in cathode designs. These factors limit the rate at which material can be sputtered off the target source. Thus, an improvement in transport efficiency allows for an improvement in coating rate, within the above limitations and consequently results in coating process productivity improvements.
Fifthly, sputter coating quality is frequently impaired by the incorporation of atmospheric gases such as oxygen, nitrogen and water vapor. This gas incorporation rate is dependent upon the relative arrival rates of gas atoms versus the sputtered atoms at the substrate. A high sputtered atom arrival rate therefore favors increased coating purity.
The low rate of arrival of sputtered atoms requires that very stringent measures be taken in the design, construction, and vacuum pumping of the sputtering apparatus to assure lower atmospheric gas impurity arrival rates. These measures lead to equipment cost increases, and usually also imply additional maintenance and operating costs. In many instances sputter coating quality is limited by the available state-of-the-art vacuum technology and practice.
U.S. Pat. Nos. 4,428,816 to Class et al and 4,472,259 to Class et al disclose magnetron sputtering cathodes having improved transfer efficiency. Such cathodes are suitable for the coating of substrates only when the substrate is caused to travel past the cathode in a linear fashion. Such cathodes are suitable therefore only in a sputtering apparatus which includes a scanning mechanism. The absence of such substrate motion, results in unacceptable thickness non-uniformity of the substrate coating.
There are reasons why the requirement for substrate translational motion is a disadvantage. First among these is the fact that many coating processes, including "planarized" aluminum coatings require close control of substrate temperature, and in addition require the application of radio frequency (RF) substrate bias power during the coating process. The application and control of substrate heating combined with the requirements of Rf power plus translation pose serious design and handling problems which increase equipment cost and degraded performance reliability.
The achievement of coating thickness uniformity combined with high transfer efficiency is difficult. This is because magnetron discharges are inherently non-uniform in their degree of ionization (plasma density). Consequently, the rate at which material is emitted from the surface of the sputtering target is also non-uniform, tending to be localized in linearly continuous regions of the target known as "racetrack" regions. As the substrate is brought closer to these racetrack regions, transfer efficiency (the fraction of the target that reaches the substrate) is improved but coating thickness uniformity is impaired.
The deposition profile at a substrate that is being coated by material sputtered from a racetrack pattern of a planar target can be modeled using the Knudsen cosine law. The cosine law states that the maximum emission of sputtered material from an emitting surface occurs along the direction which is perpendicular to the surface. The emission in any direction which is inclined at an angle, .theta., with regard to the surface perpendicular, is less than this maximum emission by the cosine of the angle .theta.. For flat, disk-shaped substrates, the obvious choice for a coating source is a cathode which yields a ring shaped racetrack which is the emitting source of the coating material. Thus, the sputtering target can be a planar disk, with a circular racetrack on it, and the substrate placed nearby, with its surface parallel to the target plane, and its center coaxial with the centerline of the racetrack. The deposition properties of such ring emitting sources have been documented in the literature. See, for example, L. Holland, Vacuum Deposition of Thin Films, Chapman & Hall Ltd., London, 1963, pages 152-156 and L. Massel and R. Glang, Handbook of Thin Film Technology, McGraw-Hill, New York, 1970, pages 1-56 to 1-59.
These studies show that there is an optimum spacing between source and substrate which yields the greatest deposition rate uniformity. For separations greater than this optimum, the deposition rate is greatest at the point where the axial centerline of the target ring intercepts the substrate plane. From this maximum, the rate decreases monotonically with radial distance from the central point, yielding a dome-shaped deposition pattern. For source to substrate separation less than the optimum the deposition pattern assumes a saddle-like shape which shows a maximum at the substrate radial position equal to the ring-source radius. This maximum becomes more pronounced as the source to substrate distance is further decreased.
The practical application of these principles to a ring shaped magnetron source requires that the zone of uniform deposition be approximately 10% greater than the substrate diameter. For these reasons, the achievement of uniform deposition on a substrate having a radius R.sub.s requires that the racetrack radius be approximately 1.1 R.sub.s and the source to substrate distance be equal to approximately 1.3 R.sub.s. Such sources typically have a transfer efficiency of 15 to 18%. Such sources are therefore limited in transfer efficiency because any attempt at decreasing the source to substrate distance, results in a non-uniform, saddle-shaped deposition profile.
Another limitation associated with this magnetron sputtering configuration is the manner by which progressive erosion of the target influences both the coating thickness profile as well as the utilization efficiency of the target. It is known that the localized ion bombardment associated with the magnetron geometry causes a target surface to be locally sputtered away. Thus, the target surface develops a localized groove in the racetrack region. As sputtering progresses, this groove becomes progressively narrower and deeper. As a consequence, the angular range over which sputtered atoms leave the target surface is narrowed i.e., becomes "over cosine". Thus, the coating rate at the substrate progressively increases for those portions of the substrate immediately adjacent to the racetrack region of the target and progressively decreases elsewhere. The resulting progressive coating thickness non-uniformity limits the useful coating life of the target, with the attendant costs associated with target replacement.
This problem is accentuated by a reduction in target to substrate distance. This arises because, at the greater substrate distance, collisions between neutral gas atoms and the sputtered atoms have the effect of broadening the angular range over which the sputtered atoms travel after leaving the target surface. This gas scattering has the effect of compensating for the deposition profile narrowing associated with a deepening target racetrack groove. As the target to substrate distance decreases, the opportunity for gas also decreases, thereby accentuating the aforementioned problems of film thickness non-uniformity and target utilization.
Another undesirable aspect of the development of a deep and narrow racetrack groove is that this causes the target to become eroded through its useful thickness before an optimum volume of the target material is used to achieve substrate coating. Consequently, more frequent target replacement with its attendant cost is mandated. For these reasons, a means of achieving a broad target erosion pattern is desirable.
Another factor which enters in the practical application of ring shaped magnetron sputtering sources is related to the conformality of the coating. Semiconductor wafer substrates are not flat when viewed on a microscopic scale. Instead, they have micrometer sized features such as steps, and square sided holes which have a depth equal to the lateral dimensions. One example would be a square hole measuring one micrometer in length and width and one micrometer in depth. Another example would be a groove measuring one micrometer in width and depth, with a length measured in tens or hundreds of micrometers. There are many reasons why it is desirable that the coating should conformally cover these features, i.e. that the coating thickness at the bottom or side-wall of a step or hole be equal to that at the top. This is a problem with sputter deposition because the majority of sputtered atoms which leave the target travel to the substrate without gas scattering and in a straight line path. Furthermore, most sputtered atoms move only short distances on the substrate surface after arrival. As a consequence, if a given substrate region does not lie on a direct line of sight path to an emitting region of the target, the sputtered atoms from that region of target are shadowed and do not contribute to the build-up of a coating in the given substrate region. Alternatively, some substrate regions may have a broad line of sight view of the target and therefore achieve coating thicknesses which are much greater than the average. As a consequence, the sputter coating of a vertical step can result in a layer which is thick and overhangs the top of the step leaving only a thin coating on the sidewall and bottom of a step. Also, for substrates coated using the ring configuration, it is not unusual to find that a substrate step which faces radially outward experiences a different coating coverage from one which faces radially inward. From this perspective it is desirable that the target emitting region be very broad, thereby affording the greatest possible viewing angle to all substrate sites.
The aforementioned problems have been partially addressed by a variety of methods disclosed in prior issued patents. It has been known for some time that the shape of the magnetic field needed to establish the target racetrack has a direct influence on the width of the racetrack groove. This magnetic field is produced by opposite magnetic poles situated behind or adjacent the target. These cause arch shaped magnetic flux "lines" to emerge from the target in the vicinity of the north magnetic pole, arch over and then reenter the target in the vicinity of the south magnetic pole. Discharge electrons are trapped by the combination of magnetic field and the strongly negative potential of the target surface. This combination of electric and magnetic field also induces a sideways magnetron drift to the electrons. To prevent the loss of these electrons, the arch shaped magnetic field is made to close on itself, thereby forming an endless "tunnel" adjacent to the target face. The discharge is confined in this region and tends to be most highly ionized in those places where the arching magnetic field is substantially parallel to the target face. Accordingly, a highly arched field produces a narrow racetrack, and a gently curved arch produces a desirable broad racetrack. U.S. Pat. No. 4,162,954 to Morrison discusses these target racetrack characteristics. Similarly, U.S. Pat. No. 4,457,825 to Lamont, Jr., discloses this art as applied to a ring source. The '825 patent also discloses the use of a ring shaped target with inward sloping faces like the frustum of a cone. Recalling the cosine law, one might expect such a target to emit more sputtered material radially inward. Accordingly, with such a target one can achieve a closer target-substrate spacing without the uniformity loss associated with the saddle-shaped deposition profile.
The characteristics of such a source are described in an article by J. C. Helmer in the Journal of Vacuum Science and Technology, Second Series, Volume 4, Number 3, Part 1, May/June 1986, pages 408-412. At an operating pressure of 6mTorr argon gas, this cathode exhibits a 21.9% transfer efficiency, and yields an aluminum sputter deposition rate of approximately 1800 angstroms per minute at an applied D.C. power of 1563 watts. From the Helmer reference it may be inferred that the useful deposition zone radius, R.sub.s is yielded by a racetrack radius of equal dimension, i.e., R.sub.s =RR, and the substrate spacing is approximately, 875 R.sub.R. A closer spacing than this causes the familiar saddle shaped deposition profile and deposition uniformity loss.
Another method of achieving a broad erosion pattern on a planar target is disclosed in U.S. Pat. No. 4,444,643 to C. B. Garrett. Here the entire field forming magnet structure is mechanically moved to cause the associated racetrack discharge to continually traverse the target face. The approach has the disadvantage of requiring mechanical motion with the inherent issue of reliability. No substrate spacing is referenced in the patent. Planar magnetron devices of this class are known to require a minimum substrate spacing of 5 cm to 6 cm to minimize electron bombardment of the substrate. (See for example, the article by W. Class and R. Hieronymi, Solid State Technology, December, 1982, pages 55 to 61.) At this spacing a substantial amount of sputtered material is lost from an annular region within 6cm from the target periphery.
Yet another method for achieving a broad erosion pattern is described in U.S. Patent No. 4,401,539 to Abe et al where an auxiliary magnetic field is used to displace the position of a racetrack produced by a primary, field-producing electromagnet. The auxiliary magnetic field is achieved by the use of an electromagnet coil which shares one of the two cylindrical pole pieces of the primary, field-producing electromagnet. The primary, field-producing electromagnet, having cylindrical symmetry, causes a circular racetrack to be generated on the face of a disk-shaped planar target. The activation of a current in the auxiliary magnet field coil, having a polarity opposite to the current in the primary coil causes a reduction in the diameter of the racetrack. This diameter reduction is proportional to the magnitude of the auxiliary coil current. A programmed coil current modulation applied to this auxiliary electromagnet coil permits the achievement of a racetrack having a diameter which synchronously varies with auxiliary coil current. Accordingly, the auxiliary coil current is used to cause a broad erosion pattern on the target as well as to compensate for the coating thickness non-uniformities which develop as target erosion progresses.
The characteristics of this source are: EQU R.sub.R =0.85 R.sub.s to 1.0 R.sub.s
and target-to-substrate spacing is 1.3 R.sub.s. No transfer efficiency characteristics are available for this source, but the geometry predicts that it is between 15 and 20%. The primary benefits of this approach reside in improved target utilization and film thickness uniformity.
The aforementioned sources all have disadvantages in film step coverage because of the limited angular range with which sputtered atoms arrive at the substrate. This range is limited by the size of the target erosion zone as well as the separation between substrate and target since, for a given erosion zone width, the angular arrival range varies inversely with the spacing.
U.S. Pat. No. 4,604,180 issued to Y. Hirikawa addresses this by combining two separate concentric ring sources. The inner ring source is achieved on a disk shaped flat target, and the outer ring source is achieved on a target with an inwardly facing surface shaped as the frustum of a cone. The relative sputter emission rate of each ring source is controlled by separately applying D.C. power to each ring source. Alternately, the rings are maintained at a common potential, and the power splitting between ring sources controlled by varying the coil current in the electromagnet.
U.S. Pat. Nos. 4,606,806 to J. Helmer, 4,595,482 and 4,627,904 to D. Mintz and No. 4,569,746 to M. Hutchinson similarly disclose separately powered concentric ring sources, one planar and the other the frustum of a cone, where the magnetic field producing means are two electromagnet coils which couple to a ferromagnetic pole-piece structure. The coils share one pole piece and the coil currents are such that the magnetic flux induced by the coils is always additive in the shared pole piece. As a consequence, two separate magnetron discharges are formed; one adjacent to the planar ring shaped target, and the other adjacent to the conical ring shaped target. Deposition uniformity is achieved by separately adjusting the electrical power to the separate discharges such that at a given substrate spacing, the deposition contributions add to give the desired uniformity.
As revealed in U.S. Pat. No. 4,627,904 to D. Mintz, the power to the outer ring target is five to twelve times greater than that applied to the inner target. As will be shown in the description of the inventive cathode disclosed here, such a power splitting will only give a uniform deposition profile for a substrate radius R.sub.s when the outer ring racetrack radius R.sub.R is of approximately equal radius, and the substrate spacing is approximately equal to 0.875 R.sub.R. This is very similar to the configuration described in U.S. Pat. No. 4,457,825 to Lamont Jr. Similar transport efficiencies are therefore predicted. This configuration will not permit extended operation at closer substrate spacing because only limited means is provided for minimization of the associated pair of racetrack grooves, which at a closer spacing would result in a deposition pattern having dual peaks with an intermediate valley associated with the annular non-sputtering region between the inner and outer targets.
Another means of overcoming the shortcomings of a single ring emitting source is disclosed in U.S. Pat. No. 4,622,121 to U. Wegman, et al. Here, again, two separate annular sputtering zones are established and a means provided for adjusting the deposition from each zone in order to achieve deposition uniformity. In the preferred embodiment, this is achieved by electrically isolating and separately powering the two targets. In an alternate embodiment, separate electromagnetic racetrack zones are established, on a target or targets maintained at a common potential, and power division achieved by adjustment of the electromagnetic coil currents.
The preferred embodiment disclosed in the '121 Patent achieves broadening of the racetrack groove by an eccentric motion applied to the field forming permanent magnet structures. The alternate embodiment has no such means. In both embodiments, the outer target has an emitting surface which is vertical or almost vertically disposed relative to the substrate plane. The inner target is planar or almost planar and is oriented in the standard manner with regard to the substrate. This configuration provides a very broad angular arrival range of sputtered atoms to the substrate, which is here limited only to the non-sputtering zones along the axial centerline of the inner target, and the annular non-sputtering region between the inner and outer racetracks. Another limitation of this configuration is associated with the vertical or near vertical orientation of the outer target. This configuration causes an appreciable degree of cross sputtering from one target to another as may be predicted from the cosine law. This configuration also severely restricts the proximity of inner target to substrate. Thus, although transport efficiency is probably high, power efficiency is predictably low because of the high degree of cross sputtering. In addition to reducing power efficiency, cross sputtering has the further disadvantage of causing excessive sputtered material build-up on the non-sputtering regions of each target. Such build-up is a known source of particulate formation as well as a source of electric arcing, which is also deleterious to the quality of the sputtered coating.